1. Field of the Invention
The present invention relates to an ink composition suitable for use in ink jet recording, comprising an acetylene glycol, an alcohol adduct of ethylene oxide and/or propylene oxide, and a pyrrolidone or its derivative in a specific composition.
2. Background Art
Ink jet recording is a recording method which comprises ejecting ink as droplets toward a recording medium, regulating droplet trajectory direction and depositing the droplet onto the recording medium at its contemplated position. This method is advantageous, for example, in that a high-definition image can be recorded at a high speed in a quiet operation and color recording can easily be realized by using multicolor inks, and has been extensively used as recording means. In recent years, particularly for high-definition image record formation, the amount of ink droplet used in ink jet recording is very small, for example, about 10 pL. To this end, ink ejection holes (nozzles) of the recording head for ink ejection are formed very finely.
To ensure good print quality, a surfactant for providing penetration effect is added to the ink for ink jet recording. Nonionic surfactants are in many cases selected as the surfactant, because they can provide good record quality and a high level of effect of penetration into recording media. In particular, acetylene glycols have a very high level of penetration effect, and the claimed advantage of the acetylene glycols is excellent record quality in a small addition amount (for example, Japanese Patent Laid-Open Nos. 139964/1988 and 239068/1992).
This acetylene glycol has a high level of hydrophobicity and can offer penetration effect, but on the other hand, due to its low solubility in water, in some cases, care should be taken to avoid clogging of ink ejection holes and a deterioration in ejection stability. A proposal has been made on a method in which, in order to dissolve an acetylene glycol having low water solubility in ink, a surfactant or organic solvent, which is low in penetration but has high solubility, is added in combination with the acetylene glycol to the ink (for example, Japanese Patent Laid-Open No. 309130/2002).
However, the use of other surfactant or organic solvent as an aid in order to dissolve a large amount of the highly hydrophobic acetylene glycol into a water-based ink disadvantageously results in significantly lowered surface tension of the water-based ink. Further, the addition of a large amount of the surfactant or organic solvent often enhances attack of nozzles. When the surface tension of the ink is significantly lowered, the cohesive force of ink droplets is lowered and, consequently, the flying form of the ink droplet is likely to become unstable. The reduced amount of ink per droplet for high-definition record formation seeds small ejection energy necessary for ink ejection so that the flying form of the ink droplets becomes unstable. This further leads to a fear of causing a deterioration in record quality. Further, an enhancement in an attack on nozzles is critically undesirable since the very finely formed nozzle member is likely to cause deformation and the like by attack.
The present inventor has previously proposed an ink composition comprising a compound represented by formula (I) which will described later and a compound represented by formula (II) which will be described later (Japanese Patent Application No. 114357/2003). Advantageously, this ink has excellent ejection stability and is less likely to attack members such as a head while ensuring a high level of print quality. The specification in this proir application, however, does not disclose the weight ratio among the compound represented by formula (I), the compound represented by formula (II), and a pyrrolidone or its derivative, according to the present invention, in any part including working examples.